They came with center battery pack tap, so your only getting 18 volts in reverse. Check your batterys for condition I have seen this happen when one or more are weak. This usually happens to the first three batterys on drivers side.

You can provide full power reverse simply by moving the green cable from battery #3 positive to the F&R bottom terminal next to its other end. This is the terminal where the battery pack main 36v red cable connects to the F&R. The green cable becomes a jumper. It works.